A Pb-containing solder (for example, Sn-38Pb) has long been used as a solder alloy for soldering. However, recent concerns over lead toxicity have created a trend for Pb-free solder alloys. Sn—Ag-based solders have been widely used as such Pb-free solder alloys, and Sn-3.0Ag-0.5Cu solders, in particular, have become standard in view of solder alloy properties such as melting point, wettability, and heat-resistance fatigue characteristics.
A solder joint needs to have high mechanical characteristics and high heat-resistance fatigue characteristics in a mounted structure in which an electrode portion of an electronic component is joined to an electrode portion on a circuit board with a solder alloy (solder joint), specifically a Pb-free solder alloy, as detailed below.
A solder alloy as a constituent member of the mounted structure has a lower wetting point than the electronic components (specifically, for example, the terminal electrodes of electronic components) or the circuit board. The mechanical characteristics of the solder joint can thus suffer greatly in a high-temperature environment of, for example, about 100° C. or more.
A solder alloy also has a smaller elastic modulus than other constituent members, and is subject to localized strains due to the differences in the linear coefficients of expansion of constituent members involving temperature changes, or localized loads due to vibrations or impact. Particularly, a solder joint may crack when it is under the repeated strains due to the differences in the linear coefficients of expansion of constituent members, and this may lead to disconnection.
A solder alloy used for the mounted structure thus requires high heat-resistance fatigue characteristics against strains that repeatedly occur with temperature changes, and high mechanical characteristics, particularly, high strength and high ductility in a high temperature environment.
There is a movement to reduce the noble metal silver contained in the Sn—Ag-based solder alloy. However, use of silver is still pervasive because low Ag contents can lead to an increased melting point, and low wettability, particularly poor heat-resistance fatigue characteristics at the solder joint.
Under these circumstances, Japanese Patent No. 3363393 discloses a Pb-free solder alloy containing no silver, specifically a Pb-free solder alloy in which 1 to 15 weight % of Bi, and 1 to 15 weight % of In are added to an alloy containing 0.1 to 2.5 weight % of Cu, and the remainder Sn, preferably with at least one selected from the group consisting of Ni, Ge, Pd, Au, Ti, and Fe.
The Pb-free solder alloy of the foregoing related art contains only Cu, Bi, and In as essential components, and is described as improving heat-resistance fatigue characteristics with addition of 0.1 to 2.5 weight % of Cu, lowering the melting point of a Sn—Cu-based solder alloy with addition of 1 to 15 weight % of Bi and 1 to 15 weight % of In. The related art also describes improving mechanical strength with addition of at least one selected from the group consisting of Ni, Ge, Pd, Au, Ti, and Fe. In the Examples section of the foregoing related art, cracking is reduced after 960 to 1,380 cycles in a heat resistance fatigue test conducted at a temperature as high as 125° C. with the Pb-free solder alloy soldering a QFP (Quad Flat Package) having interconnection leads to the land of a printed board.